Magnetic random access memory (MRAM) is a type of memory device containing an array of MRAM cells that store data using their resistance values instead of electronic charges. Generally, each MRAM cell includes a magnetic tunnel junction (MTJ) structure. The MTJ structure typically includes a stack of magnetic layers having a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric, e.g., an insulating tunneling layer. A top electrode and a bottom electrode are utilized to sandwich the MTJ structure so electric current may flow between the top and the bottom electrode.
One type of MRAM cell is spin-transfer-torque magnetic random access memory (STT-MRAM). In such a fabrication process flow, a stable magnetic tunnel junction (MTJ) stack is needed to sustain high temperature backend thermal processing while still producing high tunnel magnetoresistance (TMR) ratio. MTJ stack often starts with a buffer layer to improve adhesion and the seeding of the subsequent layers. MTJ stack also includes a synthetic ferrimagnet (SyF) coupling layer to couple the first pinning layer and the second pinning layer antiparallelly. A capping layer is utilized on top of the MTJ stack that ends with a noble metal material, which protects the stack from corrosion and also acts as a etch stop layer for hard mask etching.
During manufacturing of conventional STT-MRAM devices, a thermal annealing process is often performed right after the film layer deposition process to assist crystallization of the ferromagnetic layers as well as the insulator material sandwiched in the device structure. Insufficient thermal energy or inaccurate temperature control during the annealing process may cause the film bonding structures or properties formed in an undesirable manner. For example, inaccurate temperature control or undesired drift of the thermal diffusion during the annealing process may result in insufficient crystallization of the film layer, leading to failure of the device to meet its intended performance.
Conventional methods use a Ta- and/or Ru-based buffer layer for purposes adhesion and seeding of the subsequent layers. However, the buffer layer is easy to be segregated. The texture from the bottom contact tends to affect the texture of the MTJ film stack through the buffer and becomes detrimental to TMR ratio and magnetic properties of the stack, i.e., the texture roughness is carried from the substrate/bottom contact layers to the other layers of the MTJ film stack. Ru is also used in conventional methods for fabricating SyF coupling layers and capping layers. However, Ru tends to diffuse towards the MgO based tunnel barrier layer to bond with oxygen ion. Such detrimental diffusion lowers the TMR ratio of the film stack. The effect becomes severe during thermal process at elevated temperatures.
Therefore, there is a need in the art for improved methods and apparatus for fabricating MTJ structures for STT-MRAM applications with high volume manufacturability. There is also a need for improved MTJ stacks that are able to sustain high temperature thermal processing while preserving high TMR ratio and magnetic properties such as high SyF coupling, high perpendicular magnetic anisotropy of pinned layers and reference layer, and controllable perpendicular magnetic anisotropy of free layer.